Understanding and Tuning Carburetors - Fueling Around

What’s inside your carburetor and how to tune it

For many enthusiasts, the carburetor is some sort of black magic device that feeds an engine fuel to run. Short of bolting it on and maybe turning the base idle screw to keep the car running, few really know what's going on inside it. Out of the box, a carburetor can be close, especially if you have a mild or common engine combination. However, introduce big cams, stroker engines, large single-plane intake manifolds, and so forth, and you're going to have to tweak your carburetor's settings to not only have the carburetor perform better, but quite possibly to just get the engine to run properly. This article aims to provide you with a better understanding of the venerable carburetor, and hopefully inspire you to go ahead and perform some fine tuning yourself.

There are dozens of carburetor types to pick from and several companies that sell performance carburetors for mild-street to all-out-race combinations. We'll do our best to cover the major offerings and the most popular carburetors and how they work, but there's no denying the fact that a good book, tuning manual, or video series (like Holley's tuning DVD) is ideal for learning the complete ins and outs of your particular carburetor to get the most from it and your engine combination.

Before we delve into the major carburetor brands and design functions, we felt it best to go over some general carburetor information. Beginning with the carburetor itself, the typical unit is devised of just a few main parts; the throttle plate (also called the baseplate), the main body, and depending upon the design, there could be a separate fuel bowl or separate air horn/choke plate assembly. We'll go more into these pieces with each manufacturer in the captions.

The carburetor's three main functions are to mix air and fuel and deliver the combustible mixture to the engine, control the engine's speed, and adjust the air/fuel ratio. Fuel is delivered to the carburetor via the fuel pump, either mechanical or electric, and stored in the carburetor's fuel bowl(s). Airflow through the carburetor is created by vacuum from the engine's pistons' downward stroke, which draws air through the carb, intake, and finally into the combustion chambers. The amount of fuel drawn into this airflow is controlled by the carburetor's venturi and choke plate. The air speed increases through the venturi, creating a pressure drop that allows the fuel to be pulled into the air stream from the carburetor and ultimately into the engine. Since atmospheric pressure is a critical player in carburetor function and tuning, it is noteworthy to remember that what works for a carburetor at sea level will not be the same for a carburetor employed at 4,000 feet above sea level.

Seeing the calculated cfm is 767, a 750-cfm four-barrel carburetor would seem the logical choice. One further thing to consider is that the above calculation does not adjust for volumetric efficiency (VE) into consideration. An engine's VE rarely achieves 100 percent unless it is supercharged or turbocharged. A typical production engine will have a VE of around 70-75 percent, while a performance engine's VE will be approximately 10 percent higher. If we recalculate the cfm requirements with VE in mind, we'll get a truer requirement for our carburetion. So, if we consider our 408ci Windsor a “performance” engine with 85 percent VE, the actual cubic inches of airflow is now 347 cubic inches. If we perform the above calculations again with our new value, it will look like this: 347 (cid) x 6,500 (rpm) / 3,456 = 654 cfm

So you can see that our paper tiger 408ci Windsor would be very happy with a 650-cfm four-barrel instead, and the 750-cfm unit would be over-carbureting the engine for most applications short of high-rpm racing use. You can see these numbers in real world applications if you consider the original carburetor requirements of some of Ford's factory V-8 engines. For a similar example, the 428ci FE used a 600-cfm of carburetor and many 289ci small-blocks used just a 480-cfm unit.Even though we've done the basic airflow calculations, there are other things to consider, such as cam lift and duration, axle gearing, manual or auto trans, vehicle weight, and more. Carefully consider your engine's airflow needs with these factors in mind, and speak with the tech lines of the carburetor companies. One of the most common carburetor issues they all see is too large of a carburetor being used for the application.

Once you've determined the cfm of your carburetor, there are several other functions/features to consider. Arguably the most important feature after cfm is whether you need/want a vacuum-secondary or mechanical-secondary-based carburetor. In a nutshell, mechanical secondaries are controlled by a mechanical connection to the primary throttle bore linkage, whereas the vacuum-controlled secondary opens the secondary throttle bore via a vacuum signal related to engine load. Both styles can be tuned/adjusted to fit a vehicle's weight, operating range, and so forth, though many camps agree a vacuum-secondary carb is preferred for street use and/or with an automatic transmission. It's common to see a mechanical-secondary carb in the wrong application giving the operator poor fuel economy, hesitation on hard acceleration, and unacceptable driveability on the street. Further considerations need to be made before purchase such as linkage type, electric or manual choke, fuel inlet design, and more. Once again, we strongly recommend contacting a manufacturer's tech line for suggestions.

Choosing the proper carburetor for your application should be your first consideration, as you'll find many carburetors with similar features (vacuum secondaries, electric choke, and more) in numerous cfm (cubic feet per minute) offerings. For example, you'll find Holley 4150 four-barrel carburetors available from 390-cfm to 1,000-cfm, depending upon model sub-series. What's the best fit for that 408ci 351W stroker you just built? It's probably safe to say the 390-cfm unit is too small and the 1,000-cfm unit is too big, but what's the ideal carburetor sizing for your new engine? Is it 600-cfm, or perhaps 750-cfm? Here's a bit of basic math to get you in the ballpark.

Too small or too large of a carburetor is something you simply can't “tune around,” so to determine the cfm requirement of your engine, take the cubic inch displacement and multiply it by the maximum rpm of the engine, and then divide by 3,456. The calculation will look like this example for our hypothetical 408ci Windsor:408 (cid) x 6,500 (rpm) / 3,456 = 767 cfm